The concept of Self-Optimizing Networks (SONs) in Long Term Evolution (LTE) has been proposed recently as a means to dynamically optimize the network performance, to minimize manual configurations and to reduce the overall cost of operating a network. Optimizing radio network configuration is an important task to increase the network efficiency and to improve user performance at the same time.
The main motivation for utilizing SONs basically resides in the large number of complex network parameters as well as the high number of base stations (or cells defined by the base stations) that need to be managed and configured with minimized human interaction. There are multiple parameters that can be optimized in a radio network, comprising various cell parameters, such as antenna settings (e.g., tilt and/or azimuth), radio network parameter settings (handover, load balancing, interference coordination, etc.), scheduler parameter setting, etc. The aim of the SON concept is to optimize these parameters, some of which will exemplarily be described herein below in more detail.
The handover (HO) procedure is one of the most important functionalities of a mobile communication system. In LTE, unlike in Wideband Code Division Multiple Access (WCDMA), there is no soft handover support, and at each handover the user context, including user plane packets and control plane context, need to be relocated from one eNodeB (eNB) to the other.
It is an option whether the full protocol status of the source eNB is transferred to the target eNB or the protocols are reinitialized after the handover. The standardized X2 interface enables to transfer both the control and user plane messages and the user plane data by enabling packet forwarding between the source and target eNBs. The HO decision is based on the UE Radio Resource Control (RRC) measurement report (so-called A3 events).
Electrical antenna tilt is another important cell parameter which can be optimized in a radio network. This parameter is usually determined at time of network planning based on the support of various network planning tools. Later it can be tuned based on, for instance, drive tests in the network area or on the recently introduced concept of Minimization of Drive Tests (MDT) from 3rd Generation Partnership Project (3GPP), which allows configuring regular UEs to perform certain measurements and collect these measurements in the Operation and Maintenance (OAM) system for various network optimization purposes.
Load balancing (LB) is one of the SON functions in LTE aiming to offload overloaded cells (e.g., with high traffic demand) to less loaded neighbor cells. LB shares for example the load of the radio links among cells based on longer time scale statistics (hourly or daily basis) by changing the cell borders. LB can be configured either with HO parameter settings or with antenna settings. In 3GPP, the HO offset parameter can be used for this purpose on a cell level which is triggered when the HO measurement report is performed (and thereby implicitly triggers the actual HO as well), although it is also possible to manage handovers for example per user equipment (UE).
The LB processing can be run for example in the so-called Network Management System (NMS) for centralized solutions and also in the so-called Network Element (NE) for distributed solutions. However, if short time scale LB methods are to be planned for NE (e.g., in the eNodeB) implementation, the two LB schemes (long scale and short scale) may coexist and inter-work using for example the standardized interface Interface-N.
FIG. 1 shows an exemplary context of a Radio Access Network (RAN) 100. It is to be noted that in the following, doubled reference signs (such as 1003, 1004) designate the corresponding component to belong to a first cell and/or a second cell (see FIG. 2 ff). The RAN 100 comprises a Transport Network (TN) 1003, 1004. For example, the TN may be realized via an S1 interface in LTE or as an Iub interface in High Speed Packet Access (HSPA). The RAN 100 further comprises a Radio Base Station (RBS) site 1001, 1002 for providing base station functionality and a Switch Site 1005 for providing access to, for example, the Internet. In turn the RBS site 1001, 1002 may be comprised of one or more of a Base Station (BS), a Base Transceiver Station (BTS), a NodeB and an eNB, and an Internet Protocol (IP) RAN for interfacing between the TN 1003, 1004 and the B(T)S/(e)NB. Further, the IP RAN may comprise a Radio Bearer Control (RBC) Site for providing functionalities such as Ethernet switching, IP routing and security.
For the purpose of this entire description, the terms BS, BTS, NodeB and eNB may be used interchangeably for providing a mobility anchor for any UE camping in the B(T)S/(e)NB. As long as the mobility anchor function is fulfilled, the terms BS, BTS, NodeB and eNB only mean implementation of substantially the same mobility anchor function in different environments.
The TN 1003, 1004 may be implemented as a so-called Mobile Backhaul involving cable-bound (e.g., via wire, such as a copper line, or a fiber line) or wireless (e.g., via microwave) coupling into a so-called Metro Ethernet. Lastly, the Switch Site 1005 serves for interfacing between the TN 1003, 1004 and the Internet and/or a Public Switched Telephone Network (PSTN), and may comprise another IP RAN, which IP RAN in turn may comprise a Base Station Controller (BSC)/Radio Network Controller (RNC) site. The BSC/RNC Site may provide functionalities such as network synchronization, Ethernet switching, IP Routing and security.
FIG. 2 shows a first approach for LB. The above-described network 100 may comprise a first cell (Cell-A, 1001) and a second cell (Cell-B, 1002). Note that the terms “cell” and “Base Station” (defining the cell) may be used interchangeably. Cell A 1001 and/or cell B 1002 may comprise the RBS Site described in conjunction with FIG. 1 (if necessary). Further, whereas the BSs/cells are depicted by an antenna pole/tower, this does not foresee any particular implementation.
Cell/BS A 1001 has a default coverage 101 (depicted by a solid line), and cell/BS B 1002 has a default coverage 102 (depicted by a solid line). As shown in FIG. 2, cell/BS A 1001, in the default coverage, may anchor only one UE (UE6), whereas cell/BS B 1002 may have to anchor five UEs (UE1 to UE5).
LB may be employed as a means to balance resources among entities if the following conditions hold:                The air interface of cell/BS B 1002 is overloaded.        No transport network bottleneck in cell/BS A 1001 arises, since for example a cable interface is used.        
If both conditions are fulfilled, cell/BS A 1001 may be re-configured to extend its coverage 101 (see dashed line). In this case, cell/BS B 1002 may be enabled to offload at least two UEs (UE4 and UE5) to cell/BS A 1001. Accordingly, after LB, each cell/BS 1001, 1002 would equally anchor three UEs (UE4 to UE6 for cell A 1001, and UE1 to UE3 for cell B).
FIG. 3 shows a second approach for LB. In FIG. 3, in a default coverage, a macro cell/BS 1001 may have to anchor all UEs (UE1 to UE9). Offloading the macro cell/BS 1001 may be performed, for example, with micro sites (as provided, e.g., in so-called HetNets). Accordingly, a micro cell/BS 1002 may assist in absorbing high capacity demands if the following conditions are true:                The air interface of the macro cell/BS 1001 is overloaded.        No transport network bottleneck in the micro cell/BS 1002 arises, for example since a cable interface is used.        
If both conditions are fulfilled, the micro cell/BS 1002 may be re-configured to overtake all UEs in the coverage of the micro cell/BS 1002. In this case, the macro cell/BS 1001 may be enabled to offload at least four UEs (UE1 to UE4) to the micro cell/BS 1002. Accordingly, after LB, the load would be more evenly distributed (UE1 to UE4 for the micro cell/BS 1002, and UE5 to UE9 for the macro cell/BS 1001).
FIG. 4 shows a third approach for LB, wherein the left-hand part of FIG. 4 shows the situation before LB, and the right-hand part of FIG. 4 shows the situation after LB. As shown in FIG. 4, before LB, cell/BS B 1002 is congested with anchoring three UEs (UE1 to UE3), whereas cell/BS A 1001 has to anchor only one UE (UE4). Accordingly, TN LB performs load balancing among TN links by rerouting some part of the traffic of the overloaded TN link (of cell/BS B 1002) toward the less loaded TN link (of cell/BS A 1001). In this case, the situation after LB may reside in having added an extra direct transport link between the cell/BS 1002 with overloaded TN link and the neighbor cell/BS 1001 with less loaded TN link.
However, the third approach can be extremely costly due to the additional TN link required. Furthermore, using this approach, the air interface can still remain unutilized. In this case, the following conditions hold:                The air interface of the cell/BSs 1001, 1002 are NOT overloaded.        Transport network links constitute a bottleneck, since interfaces with low throughput are used (e.g., copper line and/or microwave).        
The above third approach has the drawback of requiring the extra TN link between the cells/BSs 1001, 1002 in order to reroute the traffic of at least some UEs. In addition, the deployment of the extra TN link may not be feasible due to practical constraints (e.g., license is denied to deploy the new cable or microwave link).
Accordingly, the TN remains often as a bottleneck, even today. In case of TN congestion, TN congestion control may determine the resource balancing, and not the air interface scheduling. In the LTE TN (implemented, e.g., as the S1 interface), the end-user Transport Control Protocol (TCP) congestion control is used as a congestion control mechanism. If TN remains a potential bottleneck, then a need for a solution to handle this bottleneck arises. In this case, TN congestion control has to be able to realize the desired resource balancing. For instance, operators may need to be supported in LTE over a TN involving 8 T1 trunks (resulting in only ˜12 Mbps TN capacity).